The present invention relates generally to signal processing systems, and more particularly to signal detection and estimation in wireless communication systems using space-time processing techniques.
A typical wireless communication system generally comprises three main components: the transmitter, the channel, and the receiver. For instance, in a wireless digital communication system, the transmitter may process digital data from a data source, pass it through a pulse-shaping filter, and then use it to modulate a carrier signal. Then for transmission, the modulated carrier is further passed to the channel. At the receiver, the output of the channel is demodulated with the same carrier signal thus forming the baseband signal. The baseband signal is passed through a matched filter and then sampled at the symbol rate. These samples are then forwarded to a decision logic to determine the received symbols, providing the original digital data.
While in the channel, the signal is distorted and corrupted due to various phenomena, and may have significantly degraded before arriving at the receiver. Among the main reasons for this are: inter-symbol interference (ISI), fading, co-channel interference (CCI), and thermal (white) noise. The inter-symbol interference results when the data symbols contained within a data stream interfere with one another, i.e. due to undesired interaction of a wireless channel with itself. This happens because of signal bandlimiting by the channel and/or the shaping filter, and/or because the wireless channels often become multipath channels for a variety of reasons such as the presence of obstacles in the line-of-sight. Multipath also causes fading.
The co-channel interference results when two or more users operate on one radio frequency. It is very common in cellular phone systems where the same frequencies are recycled from cell to cell. Unfortunately, the transmitted signals tend to undesirably travel to non-targeted proximate cells, causing interference with the signals within a non-targeted proximate cell. Thermal (white) noise is always present in electronic devices.
Several techniques may be used to cope with such undesired effects. Essentially, for compensating the inter-symbol interference, equalization is often performed in a variety of communication systems; for compensating fading, diversity techniques are generally employed; and white noise effect is minimized by use of matched filters. On the other hand, depending on the strength of each user's signal, the co-channel interference may potentially impede accurate reception of the concerned user's data. For a particular user, any number of interference-causing sources may be located at different locations, making it extremely difficult to provide a reasonable compensation through a simple combination of the above-mentioned techniques. In addition, inconsistent behavior of the interfering sources located at unknown origins with respect to the receiver, including on/off switching, may cause the various interferences to constantly change, as an example. A fixed compensation system may fail to adequately compensate for time-varying interference, since it may not be able to follow these time-variations and remove the varying interference.
Thus, in wireless communication systems, a more effective compensation mechanism is desirable to cope with above-mentioned undesired effects.